System and method for matching electrode resistances in OLED light panels

ABSTRACT

Provided are an OLED device and a method of manufacturing the OLED device that may provide improved luminance uniformity. The disclosed OLED may have a first electrode that has a first sheet resistance Rs, and a second electrode that has a second sheet resistance, wherein the second sheet resistance may be in the range of 0.3 Rs-1.3 Rs. In addition, the disclosed OLED may have a plurality of equal potential difference between points on a first electrode and a second electrode. The equal potential difference may be provided by a gradient resistance formed on at least one of the electrodes.

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Regents of the University of Michigan,Princeton University, The University of Southern California, and theUniversal Display Corporation. The agreement was in effect on and beforethe date the claimed invention was made, and the claimed invention wasmade as a result of activities undertaken within the scope of theagreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices and,more specifically, to devices that may have matched electroderesistances that provide substantially uniform luminance.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as fiat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

A ligand may be referred to as “photoactive” when it is believed thatthe ligand directly contributes to the photoactive properties of anemissive material. A ligand may be referred to as “ancillary” when it isbelieved that the ligand does not contribute to the photoactiveproperties of an emissive material, although an ancillary ligand mayalter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled inthe art, a first work function is “greater than” or “higher than” asecond work function if the first work function has a higher absolutevalue. Because work functions are generally measured as negative numbersrelative to vacuum level, this means that a “higher” work function ismore negative. On a conventional energy level diagram, with the vacuumlevel at the top, a “higher” work function is illustrated as furtheraway from the vacuum level in the downward direction. Thus, thedefinitions of HOMO and LUMO energy levels follow a different conventionthan work functions.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

SUMMARY OF THE INVENTION

In an embodiment, a light-emitting device having improved luminanceuniformity is provided. The device may include a first electrode havinga first electrical sheet resistance Rs and a second electrode having asecond electrical sheet resistance in the range of about 0.3 Rs-1.3 Rs.Preferably, the second electrical sheet resistance may be in the rangeof 0.7 Rs-1.0 Rs. An organic emissive layer is disposed between thefirst electrode and the second electrode. A plurality of organic lightemitting devices may be arranged in series.

In an embodiment, an organic light emitting device may include a firstelectrode, a second electrode, and an organic emissive layer disposedbetween the first electrode and the second electrode. A gradientresistance of at least one of the first and second electrodes is in acurrent flow direction.

In an embodiment, an organic light emitting device may include a firstelectrode, a second electrode, and an organic emissive layer disposedbetween the first electrode and the second electrode. A potentialdifference between each of a plurality of points on the first electrodeand a corresponding point on the second electrode measured in adirection substantially perpendicular to the first electrode and to thesecond electrode, is within 5% of each other.

Methods of fabricating organic light emitting devices are also provided.For example, in an embodiment, a device may be fabricated by obtaining afirst electrode source material and disposing the material over asubstrate. The substrate may be arranged at an angle θ relative to aline normal to the substrate from the first electrode material source,where 0<θ<90°. The first electrode material may be deposited onto thesubstrate to firm a first electrode having a non-uniform thickness.

In embodiment, fabrication of an organic light emitting device mayinclude a plurality of layers of an electrode material that is depositedthrough a series of shadow masks. Each shadow mask in the series ofshadow masks may have a smaller area through which the electrodematerial is deposited than the previous shadow mask in the series.

According to an embodiment, an organic light emitting device may befabricated by passing a substrate through a plurality of positions in alinear deposition system. At each position, electrode material may bedeposited over a substrate at a different thickness and over a differentportion of an area of the substrate than at each of the other positions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device (OLED).

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows electrical current flow through an OILED according to anembodiment of the present invention.

FIG. 4A shows a view of OLED electrodes for providing an equipotentialsurface according to an embodiment of the present invention.

FIG. 4B shows a schematic representation of expected potentials atpoints between an anode and a cathode of a conventional OLED device.

FIG. 4C shows a schematic representation of expected potentials atpoints between an anode and a cathode of an OLED device according to anembodiment of the present invention.

FIG. 5 shows a cross-sectional view of an OLED according to anembodiment of the present invention.

FIG. 6A provides a functional illustration of a deposition systemutilizing masks for fabricating a graded electrode structure on an OLEDaccording to an embodiment of the present invention.

FIG. 6B shows a top view of an electrode according to an embodiment ofthe present invention.

FIG. 7 shows an example of a layer deposition system for fabricating anOLED according to an embodiment of the present invention.

FIG. 8 shows an equivalent circuit of the OLED device shown in FIG. 3according to an embodiment of the present invention.

FIG. 9 shows simulation results of luminance distribution in 1-D OLEDdevice with uniform cathode according to an embodiment of the presentinvention.

FIG. 10 shows a schematic drawing of panel layout according to anembodiment of the present invention.

FIGS. 11A-C show device structures for Panels A, B and C according to anembodiment of the present invention.

FIG. 12 shows a photo-like image of Panels A, B and C according to anembodiment of the present invention.

FIG. 13 shows a normalized luminance distribution of Panels A, B and Caccording to an embodiment of the present invention.

FIG. 14 shows a bar chart of cathode resistance as a function of thedistance in order to achieve equal potential across the device accordingto an embodiment of the present invention.

FIG. 15 shows sheet resistance distributions for both anode and cathodein a 4-segment case according to an embodiment of the present invention.

FIG. 16 shows simulated luminance distributions of an OLED panel withuniform cathode and 4-segment cathode according to an embodiment of thepresent invention.

FIG. 17 shows an equivalent circuit diagram for an OLED with varied HTLthickness according to an embodiment of the present invention.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton, ” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151154, 1998; (“Baldo-I”)and Baldo et al., “Very high-efficiency green organic light-emittingdevices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75,No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference intheir entireties. Phosphorescence is described in more detail in U.S.Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference.

FIG. 1 shows an organic light emitting device 100. The FIGs. are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, a cathode 160, and a barrier layer 170.Cathode 160 is a compound cathode having a first conductive layer 162and a second conductive layer 164. Device 100 may be fabricated bydepositing the layers described, in order. The properties and functionsof these various layers, as well as example materials, are described inmore detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which areincorporated by reference.

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is m-MTDATA dopedwith F.sub.4-TCNQ at a molar ratio of 50:1, as disclosed in U.S. PatentApplication Publication No. 2003/0230980, which is incorporated byreference in its entirety. Examples of emissive and host materials aredisclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which isincorporated by reference in its entirety. An example of an n-dopedelectron transport layer is BPhen doped with Li at a molar ratio of 1:1,as disclosed in U.S. Patent Application Publication No. 2003/0230980,which is incorporated by reference in its entirety. U.S. Pat. Nos.5,703,436 and 5,707,745, which are incorporated by reference in theirentireties, disclose examples of cathodes including compound cathodeshaving a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thetheory and use of blocking layers is described in more detail in U.S.Pat. No. 6,097,147 and U.S. Patent Application Publication No.2003/0230980, which are incorporated by reference in their entireties.Examples of injection layers are provided in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety. A description of protective layers may be found in U.S. PatentApplication Publication No. 2004/0174116, which is incorporated byreference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the stricture of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly For example, in device 200, hole transportlayer 225 transports holes and injects holes into emissive layer 220,and may be described as a hole transport layer or a hole injectionlayer. In one embodiment, an OLED may be described as having an “organiclayer” disposed between a cathode and an anode. This organic layer maycomprise a single layer, or may further comprise multiple layers ofdifferent organic materials as described, for example, with respect toFIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et at, which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,(preferred methods include thermal evaporation, inkjet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as inkjet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the presentinvention may further optionally comprise a barrier layer. One purposeof the barrier layer is to protect the electrodes and organic layersfrom damaging exposure to harmful species in the environment includingmoisture, vapor and/or gases, etc. The barrier layer may be depositedover, under or next to a substrate, an electrode, or over any otherparts of a device including an edge. The barrier layer may comprise asingle layer, or multiple layers. The barrier layer may be formed byvarious known chemical vapor deposition techniques and may includecompositions having a single phase as well as compositions havingmultiple phases. Any suitable material or combination of materials maybe used for the barrier layer. The barrier layer may incorporate aninorganic or an organic compound or both. The preferred barrier layercomprises a mixture of a polymeric material and a non-polymeric materialas described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporatedby reference in their entireties. To be considered a “mixture”, theaforesaid polymeric and non-polymeric materials comprising the barrierlayer should be deposited under the same reaction conditions and/or atthe same time. The weight ratio of polymeric to non-polymeric materialmay be in the range of 95:5 to 5:95. The polymeric material and thenon-polymeric material may be created from the same precursor material.In one example, the mixture of a polymeric material and a non-polymericmaterial consists essentially of polymeric silicon and inorganicsilicon.

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, medical monitors, televisions,billboards, lights for interior or exterior illumination and/orsignaling, heads up displays, fully transparent displays, flexibledisplays, laser printers, telephones, cell phones, personal digitalassistants (PDAs), laptop computers, digital cameras, camcorders,viewfinders, micro-displays, vehicles, a large area wall, theater orstadium screen, or a sign. Various control mechanisms may be used tocontrol devices fabricated in accordance with the present invention,including passive matrix and active matrix. Many of the devices areintended for use in a temperature range comfortable to humans, such as18 degrees C. to 30 degrees C., and more preferably at room temperature(20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, arylkyl,heterocyclic group, aryl, aromatic group, and heteroaryl are known tothe art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32,which are incorporated herein by reference.

as used herein, “luminance uniformity” refers to the ratio between theminimum luminance and the maximum luminance. Thus, a panel that has thesame luminance across the panel will have a luminance uniformity of100%. It is known that when scaling up to large size, the luminanceuniformity of an OLED panel degrades across the panel. The luminancenon-uniformity occurs because conventional transparent conducting oxides(TCOs), such as ITO, and IZO, have relatively high sheet resistance,varying from 10 to 100 Ω/□ (ohms/sq), which causes resistive loss, i.e.potential drop, as current flows across the panel. Luminancenon-uniformity may further cause non-uniform aging, which may lead to areduced device lifetime. One technique to address this challenge is toembed highly conductive metal bus lines on to the electrode to promote auniform current distribution. However, the use of bus lines may reducethe fill factor of the OLED panel due to the non-emissive property ofmetal. In addition, the process steps are more complicated whenintegrating bus lines, which adds to the manufacturing cost and thetotal average cycle time (TACT) of manufacturing the device.

Embodiments of the present invention may provide a device that outputslight having improved luminance uniformity, e.g. >80% at 1,000 cd/m². Asused herein, luminance uniformity may be used to compare the brightnessof various positions of the light source, i.e. OLED panel in thisinvention, and does not take chromaticity into account. That is to say,optical distortion, or color variation, may exist even when theluminance uniformity is high.

In some embodiments, the OLED device may have a more uniform potentialdrop across the two electrodes, and thus an improved luminanceuniformity. This may be achieved by matching the sheet resistances ofthe two electrodes. FIG. 3 shows electrical current flow through an OLEDaccording to an embodiment of the present invention. The OLED device 300may include an anode layer 310, a layer or layers of organic material320 and a cathode layer 330. Anode and cathode drive contacts 312, 332may be on opposite ends of the device.

In a conventional bottom-emission OLED, it is common practice to designthe conductance of the cathode layer 330 to be as great as possible,which means the resistance is as low as possible. Typically, the cathodeof a conventional bottom-emission OLED may be considered grounded. Thehighly-conductive cathode layer 330 is commonly formed from a metal.Conversely, the anode layer 310, which commonly includes ITO or asimilar material, may have a greater resistance or lower conductance. Byconsidering segments of the device (represented by dash lines)individually, each single segment may be considered as an individualOLED. The current I (shown by dark arrows) may flow through oneelectrode contact 312 of anode 310 (e.g. the first electrode) and eachof the segments of the organic stack 320 to the opposite electrodecontact 332 of cathode 330 (e.g. the second electrode). In aconventional configuration, the luminance typically is greatest in thesegment closest to the anode contact 312 and lowest at the segmentfarthest from the anode contact 312, which happens to be the segmentclosest to the cathode contact 332. The difference in luminance is dueto a greater device voltage at the segment closest to the anode contact312 than at the segment closest to the cathode contact 332, which is thesegment farthest from the anode contact 312. As a result, conventionalOLED devices have non-uniform luminance across the OLED device becauseof the uneven voltage across the panel.

In contrast to conventional techniques and the accepted idea ofimproving luminance uniformity by attempting to further reduce theresistance of the cathode layer, the inventors have increased the sheetresistance of the cathode layer 330 or portions of the cathode layer 330to generate a more uniform potential across the OLED segments in theorganic layer 320. It has been found that this unexpectedly may provideincreased luminance uniformity as well as a substantial increase in fillfactor. It has also been found that matching the resistances of the twoelectrodes in an OLED unexpectedly may provide approximately a 10%increase in luminance uniformity or more, compared to a configuration inwhich the resistance of one electrode is relatively high and theresistance of the other electrode is very low.

As an example, FIG. 4A shows a cross-sectional view of an OLED deviceaccording to an embodiment of the present invention. The OLED device 400may include a first electrode 410, an organic layer 420, and a secondelectrode 130. The first electrode 410 may be an anode layer and thesecond electrode 430 may be a cathode layer. The first electrode 410 mayhave a sheet resistance Rs. For example, the sheet resistance Rs of thefirst electrode may be 10-100 Ω/□. The organic layer 420 may be formedas a single layer or a plurality of layers. The organic layer 420 may beformed from one or more organic materials to provide the designed lightproduction. For ease of explanation, the organic layer 420 is shown butthe details of the layer are not necessary for purposes of thisdiscussion and are omitted. The second electrode 430 may be formed in amanner that provides a match of potential drop between any point on thefirst electrode 410 and a corresponding point on the second electrode430. By selecting the resistances of the first electrode 410 and thesecond electrode 430 to be matched within a range of 0.30-1.30 of thesheet resistance Rs of the first electrode 410, an improved luminanceuniformity of the panel may be attained. Alternatively, the sheetresistance of the second electrode may be in the range of 0.50-1.20 ofRs. Preferably, the sheet resistance of the second electrode may be inthe range of 0.70-1.0 of Rs. More preferably, the sheet resistance ofthe second electrode may be equal to Rs.

For example, as shown in FIG. 4A, point P1 is located at (X1, Y1) on thefirst electrode 410, and point P1′ is a corresponding point on thesecond electrode located at (X1′, Y1′) on the second electrode 430,where X1 equals X1′ and Y1 equals Y1′. Electrical current may besupplied at electrode contact 412 and may disperse to all points on thefirst electrode 410. Similar to the current flow shown in FIG. 3, thecurrent flow may be generally in the direction from the electrodecontact 412 to the opposite electrode contact 432 on the secondelectrode 430. The current may pass through the organic layer 420 to thesecond electrode 430, and to the opposite electrode contact 432. As usedherein, the electrode contact 432 is “opposite” the other electrodecontact 412 because it is located on a single side of the secondelectrode at a location that is farthest from electrode contact 412 onthe first electrode as measured across the device. The oppositeelectrode contact 432 may be at a reference voltage, such as groundpotential.

The potential difference (voltage between the two electrodes) betweenpoint P1 on the first electrode 410 and point P1′ on the secondelectrode 430 may be equal to V. Another point P2 located at (X2, Y2) onthe first electrode 410 also may have a corresponding point P2′ locatedat (X2′, Y2′) on the second electrode 430. The coordinates of each pointP2 and P2′ are equal, so X2 equals X2′ and Y2 equals Y2′. The potentialdifference between point P2 and point P2′ may be equal to V′.

In a conventional bottom-emission OLED, where the cathode is highlyconductive, the cathode potential 490 shown in FIG. 4B may be consideredas ground at any point on the plane of the cathode between the anodecontact 482 and the cathode contact 492. In which case, the potential atany two points in the plane of the cathode electrode may be equal atapproximately zero potential. Therefore, cathode potential 490 is flat.The anode electrode in a conventional bottom-emission OLED may have asheet resistance that generates a potential difference between the anodeand the cathode. Therefore, the potential of any two points in the planeof the anode electrode may not be equal, and the anode potential 480 isnot flat. In fact, the farther a point is from the anode contact 482,the lower the potential will be at that point. As shown in FIG. 4B, thepotential difference V1B (closest to the anode contact 482) is greaterthan the potential difference V2B (between the anode contact 482 and thecathode contact 492), which is greater than the potential difference V3B(closest to the cathode contact 492).

Conversely, as described herein, the second electrode 430 (as shown inFIG. 4A) may be engineered so that it has a sheet resistance greaterthan zero, and therefore, the potentials of P1′ and P2′ shown in FIG. 4Aare different. By selecting the sheet resistance of the second electrodeto match the sheet resistance of the first electrode, the differencebetween V1 and V2 may be reduced to have, for example less than 5% or,more preferably, less than 1% variation, and an improved luminanceuniformity may be achieved. For example, as shown in FIG. 4C, the secondelectrode may be the cathode, and the cathode potential 430C decreasesfrom a higher potential nearest the anode contact 412C to a minimumpotential closest to the cathode contact 432C. The potential betweenrespective points of the electrodes may be represented by the anodepotential 410C and the cathode potential 430C, and may be substantiallyuniform. For example, the potential difference V4 closest to the anodecontact 412C may be substantially equal to the potential difference V5between the anode contact 412C and the cathode contact 432C. Similarly,both potential differences V4 and V5 may be substantially equal topotential difference V6 nearest to the cathode contact 432C. In otherwords, a substantially uniform potential is provided across the OLEDdevice. The improved uniform potential between the electrodes also maylead to more uniform aging across the panels of the OLED device, thusavoiding non-uniform aging, differential coloring due to aging, localhot spots, reduced lifetime and the like. Data showing specific examplesof these considerations is provided below.

The electrode contact may be positioned at single edge of the electrode.In addition, the electrode contact edge may be less than 25% of thetotal circumference of the electrode. In some embodiments, the contactsof opposite electrode may be positioned at approximately the farthestlocations across the device. The OLED device 400 may be a transparent ortop-emission OLED device in which the sheet resistance of a cathode istypically higher than a cathode in conventional bottom-emission OLEDdevices. Transparent or top-emission OLEDs may more readily benefit forthe disclosed techniques because the engineering, e.g. adding sheetresistance, of the cathode is easier to achieve. An implementation ofthe OLED device 400 may have an active area greater than 50 cm². Morepreferably, the OLED device 400 may have an active area of at least 72cm². In addition, a side of either the first or second electrode may beat least 2 cm. The resistance of the second electrode may be a functionof the thickness of the material, usually a metal, used to fabricate theelectrode. Surprisingly, in contrast to a conventional arrangement inwhich at least one electrode has as small a sheet resistance aspossible, an arrangement as described may provide improved uniformity inluminance and potential of the device.

In order to achieve a uniform potential difference between theelectrodes, the sheet resistance of the first electrode, secondelectrode or both may be manipulated by varying the electrode thicknessso that an equal potential difference is maintained across the organiclayer. FIG. 5 shows an implementation of an OLED device according to anembodiment of present invention. The OLED device 500 may include a firstelectrode 510 (an anode), a second electrode 530 (a cathode), an organicemissive layer 520 disposed between the first electrode 510 and thesecond electrode 530, and a substrate 545. The organic emissive layer520 may include a host and a phosphorescent dopant. For ease ofexplanation, only the manipulation of the second electrode will bediscussed. The thickness of the second electrode 530 may be graded to anon-uniform thickness from one side to another side along the devicelength L. For example, the second electrode 530 may have a firstthickness at a point near the electrode contact 512 and may have asecond, different thickness at the opposite electrode contact 532 asshown by grading 535. The amount of material deposited on the secondelectrode 530 may increase, or be graded, across the device 500 lengthL. The resistance gradient may be in the current flow direction withinthe electrode. Current may be applied to the OLED device 500 atelectrode contact 512 and may pass to the opposite electrode contact532. Therefore, the graded thickness 535 and resultant non-uniformresistance of the second electrode 530 may provide a relatively uniformpotential difference between the first electrode 510 and the secondelectrode 530. As previously discussed, this may provide improvedluminance uniformity.

The described structures may be particularly preferred for top-emissionand transparent OLED devices. In an embodiment, the electrodes of theOLED device may have sheet resistances that substantially match. Inother embodiments, one of the two electrodes of the OLED device may havea sheet resistance that is a gradient resistance. Although the above wasdescribed with reference to the second electrode 530 having anon-uniform resistance, the disclosed implementations should not belimited to only the second electrode. It should be understood that thefirst electrode 510 may be formed with a non-uniform resistance insteadof the second electrode.

Examples of the techniques for fabricating the disclosed structures areillustrated in FIGS. 6A, 6B and 7. FIG. 6A provides a functionalillustration of a deposition system according to an embodiment of thepresent invention. The deposition system 600 may utilize a series ofshadow masks for fabricating a graded electrode structure for an OLEDdevice. The system 600 may have a plurality of layers deposited atvarious chambers, locations, or steps in a fabrication process, such asC and D, and may include source material 650, a plurality of depositiondevices 615, 625, a plurality of masks 620C, 620D, and a transportdevice (not shown) for moving masks from step to step for fabrication.The figure shows views of a partially-built OLED device, which includessubstrate 640 and electrode 630C, 630D, respectively. At step C, mask620C may be transported over substrate 640 with electrode 630C and thedeposition device 615. At station C, layer 3 of the electrode 630C maybe deposited on electrode layer 2 that was deposited at a prior step.The spray area 616 may cover the shadow mask 620C, such that theelectrode source material 650 is deposited over layer 2 in the regiondesired to form electrode layer 3. After layer 3 is deposited, the OLEDdevice or the masks may be moved to the configuration shown in step D.Step D may be configured similar to step C except that the mask 620D maybe different.

Step D is shown with an OLED device that includes substrate 640 andelectrode 630D. The electrode 630D may, for example, be a secondelectrode as shown in FIG. 5. As the OLED device is transported fromstep C, electrode source material 650 may be deposited by the depositiondevice 625 through mask 620D to form electrode layer 4 on top of layer 3of electrode 630D. Electrode layer 4 may have a similar width as layers1-3, but may have a different length, than electrode layers 1-3 becausethe mask 620D may be different from mask 620C. Layers 1-3 may havesimilar widths to one another, but each may have a length different froma previous layer. For example, layer 1 may have a longer length thanlayer 2, which may have a longer length than layer 3.

FIG. 6B shows a top view of an electrode according to an embodiment ofthe present invention. The electrode 670 may be formed using a processas shown in FIG. 6A or any other suitable fabrication process. Theelectrode 670 may be a cathode layer. The electrode 670 may have aplurality of layers 1-4, and a cathode contact 671. As shown, the areaof each successive layer 1-4 of the electrode 670 may decrease, suchthat, for example, a layer may extend farther from the cathode contact671 than layers disposed over that layer. The physical structure of theelectrode may be thicker in the region closest to the cathode contactthan the anode contact area which may be on the opposite side of thecathode contact. This layer configuration may also provide a gradientresistance in the current flow direction across the device. Finerthickness variation may be, realized by increasing the number of layers.In other embodiments, anode resistance may be tuned, or matched, withrespect to the cathode resistance so as to achieve equal potentialsbetween the electrodes. Simulations discussed in the Experimentalsection illustrate the benefits of the illustrated arrangement. Theactual physical structure of the OLED electrode may differ from theillustrated example, and may have more or less layers to provide thedesired grading and gradient resistance. In an embodiment, the lengthsin addition to the widths of the four layers may be different.

It will be understood that the specific example shown in FIGS. 6A and 6Bis illustrative only, and other techniques may be used to fabricate anelectrode with non-uniform resistance. For example, a continuousdeposition process may be used in which the electrode has acontinually-varying thickness, such as by means of a shadow mask thathas a variable mask opening which changes over time as electrodematerial is deposited. Such a configuration may allow for fabrication ofan electrode with a gradient thickness and, therefore, a gradientresistance. Other techniques for fabricating suitable electrodes havingnon-uniform resistance are also disclosed in further detail herein.

FIG. 7 shows an example of a layer deposition system for fabricating anOLED device according to an embodiment of the present invention. Thelayer deposition system 700 may include a deposition device 715,substrate jigs 740 and additional components. The substrate jig 740 maybe designed to securely hold a substrate 730 through the depositionsystem 700 as it passes beneath different deposition devices, such asdeposition device 715. The substrate jig 740 may be arranged to hold thesubstrate 730 at an angle θ relative to a line normal to the substratefrom the first electrode material source. The angle θ may be between 0to 90° (i.e., 0<θ<90°). The angle θ may be determined to allow theappropriate amount of electrode material to be deposited so theelectrode 720 may have a gradient thickness and thus a gradientresistance suitable for OLED device design parameters. The substrate jig740 with the substrate 730 may pass beneath the deposition device 715.

Generally, the deposition device 715 may apply the electrode sourcematerial 750 to a substrate 730 to form an electrode 720 having anon-uniform thickness. Of course, multiple passes beneath the depositiondevice 715 may also occur with the electrode source material 750 beingapplied in increasing layers after the substrate jig 740 has passed thespray area 717. In addition, different masks may be used during eachpass of the substrate 730 under the deposition device 715. Modificationsor alterations to the disclosed fabrication processes within the scopeof one of ordinary skill in fabrication processes are envisioned. Forexample, a linear vacuum deposition system may be used, for example, todeposit source material with various thicknesses at different positionson the substrate when the substrate passes through the depositionchamber.

Benefits of the disclosed devices and structures may include asimplified contact layout that may be nearly one dimensional and theachievement of substantially uniform luminance. In addition, thedisclosed structures may provide OLED devices including an OLED having afill factor of up to 100%. The disclosed structures may also be favoredin a series connection configuration.

By designing a cathode resistance to match that of the anode, it hasbeen found that the OLED voltage may be more precisely controlled toachieve better luminance uniformity of the device, as explained herein.

An equivalent circuit of a typical OLED is shown in FIG. 8, where n isthe total amount of segments being modeled, such as described withrespect to FIG. 3, m represents the m^(th) segment starting from thecathode contact, R_(ai) (i=1, 2 . . . n) is the anode resistance of thei^(th) segment, R_(ki) (i=1, 2 . . . n) is the cathode resistance of thei^(th) segment, V_(Di) (i=1, 2 . . . n) is the device voltage of thei^(th) OLED (i.e., the potential difference between the two electrodesacross the organic stacks), l_(i) (i=1, 2 . . . n) is the currentflowing through the i^(th) OLED segment, and V_(i) and V_(i)′ (i=1, 2 .. . n) are the node potentials between the adjacent anode and cathoderesistors, respectively. The first OLED segment, i.e., i=1, is the oneclosest to the cathode contact, and the last OLED segment, i.e., i=n, isthe one closest to the anode contact. By designing the cathoderesistance to match that of the anode, the OLED voltage may becontrolled to achieve better luminance uniformity as described below.

A current passing through the m^(th) cathode resistor I_(k,m), may bedescribed as

$\begin{matrix}{{I_{n} = {A_{n} \cdot J_{n}}}{I_{k,n} = {A_{n} \cdot J_{n}}}{I_{k,{n - 1}} = {{I_{k,n} + {A_{n - 1} \cdot J_{n - 1}}} = {{A_{n} \cdot J_{n}} + {A_{n - 1} \cdot J_{n - 1}}}}}\vdots{I_{k,m} = {\sum\limits_{i = m}^{n}{A_{i} \cdot J_{i}}}}} & {{Eq}.\mspace{14mu} A}\end{matrix}$

where A_(i) is the area of the i^(th) OLED segment and J_(i) is thecurrent density. The current flowing through the m^(th) anode layerI_(a,m), may be described with Eq. B:

$\begin{matrix}{I_{a,m} = {\sum\limits_{i = 1}^{m - 1}{A_{i} \cdot J_{i}}}} & {{Eq}.\mspace{14mu} B}\end{matrix}$

The potential of any pair of nodes can be written as:

$\begin{matrix}{\mspace{79mu}{V_{m} = {V_{m - 1} + {R_{k,m} \cdot {\sum\limits_{i = m}^{n}{A_{i} \cdot J_{i}}}}}}} & {{Eq}.\mspace{14mu} 1} \\{\mspace{79mu}{V_{m}^{\prime} = {V_{m - 1}^{\prime} + {R_{a,m} \cdot {\sum\limits_{i = 1}^{m - 1}{A_{i} \cdot J_{i}}}}}}} & {{Eq}.\mspace{14mu} 2} \\{\mspace{79mu}{{\because{V_{m}^{\prime} - V_{m}}} = V_{D,m}}} & \; \\{{\therefore\left. {(2) - (1)}\Rightarrow V_{D,m} \right.} = {V_{D,{m - 1}} + {R_{a,m} \cdot {\sum\limits_{i = 1}^{m - 1}{A_{i} \cdot J_{i}}}} - {R_{k,m} \cdot {\sum\limits_{i = m}^{n}{A_{i} \cdot J_{i}}}}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

In the case of a typical bottom emission OLED where R_(k,m)=0,

$\begin{matrix}{V_{D,m} = {V_{D,{m - 1}} + {R_{a,m} \cdot {\sum\limits_{i = 1}^{m - 1}{A_{i} \cdot J_{i}}}}}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

This means that the OLED segments closer to anode contact are alwaysbrighter than the segments that are farther away (because V_(D) ishigher), as long as the anode is resistive. This result is well known ina typical bottom emission OLED that includes an ITO anode and a highlyconductive cathode.

Comparing Equations 3 and 4, it can be seen that with the introductionof a cathode resistance R_(k), it is possible to reduce the voltagedifference between neighboring OLED segments (e.g. V_(Dm) and V_(Dm-1)).In a standard bottom emission OLED where R_(k) is very small, the regionnear the anode contact has the highest luminance level. This can beexplained by the difference of voltages near the anode (V_(Dn)) andcathode (V_(D1)) contacts. When the cathode resistance is very low,V_(n)≈V₁. At the same time, V_(n)′>V₁′ because of the anode sheetresistance, therefore, V_(Dn)=V_(n)′−V_(n)>V₁′−V₁=V_(D1), resulting in arelatively very large difference in luminance of the i^(th) and then^(th) OLED segments. However, when a cathode resistance is introduced,V_(n)>V₁, it may enable V_(n)′−V_(n)=V₁′−V₁. As a result, the luminancenear the anode contact decreases, nevertheless, the total luminanceuniformity may be improved.

A simple configuration described above may improve luminance uniformityfor a large-area OLED device by tuning the sheet resistance of thecathode to match that of the anode. It is also possible to find an exactsolution to Eq. 3 so that equal potential may be maintained across thepanel and, therefore, 100% uniformity may be achieved. To do so, thevoltage of each OLED segment should be the same: V_(D,m)=V_(D,m-1).Equation 3 can be rewritten, for a device length L, as:

R_(a)(x) ⋅ ∫₀^(x)A(x)J(x)dx  = R_(k)(x) ⋅ ∫_(x)^(L)A(x)J(x)dx, x ϵ[0, L] 

where, x is the distance from the cathode contact. If the area of eachsegment is allowed to be the same, the current density of each OLED willbe same too, i.e., A(x)=A,J(x)=J. Thus

$\begin{matrix}{{{{R_{a}(x)} \cdot {\int_{0}^{x}{AJdx}}}\  = {\left. {{R_{k}(x)} \cdot {\int_{x}^{L}{AJdx}}}\Rightarrow{{R_{a}(x)} \cdot x} \right. = {{R_{k}(x)} \cdot \left( {L - x} \right)}}}\ } & {{Eq}.\mspace{14mu} 5}\end{matrix}$

When the resistances of the anode and cathode meet this requirement,there will be an equal voltage and current among all the OLED segmentsin the device, which may result in 100% uniformity. This implies thatthe closer to the anode contact, the less resistive the anode materialshould be and, similarly, the closer to the cathode contact, the lessresistive the cathode material should be,

If the anode resistance is fixed at R_(a)(x)=R_(a), the requirement forthe cathode resistance to achieve an equal potential (and thus 100%uniformity) may be described as

$\begin{matrix}{{{R_{k}(x)} = {R_{a}\frac{x}{L - x}}},{x\;{\epsilon\left\lbrack {0,L} \right\rbrack}}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$

For R_(a)=15 ohm/sq and L=100, the cathode resistance as a function ofthe distance can be plotted as shown in FIG. 14. FIG. 14 shows that thedependence of R_(k) on distance is not linear. Instead, it has a gradualrise between zero and about 50 Ω/□ for up to 80% of the total length,and a dramatic rise from 50 Ω/□ to infinite in the last 20% length ofthe device. Similarly, if the cathode resistance is fixed, the anoderesistance may be tuned to meet the requirement. Or, both the cathodeand anode resistances may be varied at the same time.

Numerically, Eq. 6 can be written as follows:

$\begin{matrix}{R_{k,m} = {{R_{k}(m)} = {R_{a}\frac{m - 1}{n - m + 1}}}} & \;\end{matrix}$

The case of a 4-segment (m=1, 2, 3, 4) OLED device with n=100 wassimulated. In the simulation the anode resistance was maintained at 15Ω/□, and the cathode sheet resistance was given 4 different valuesdepending on the position. The resistance distributions of bothelectrodes are shown in FIG. 15. It can be seen that in this case, thecathode sheet resistance of the last segment is 195 Ω/□. Typically, a 20Å thick vacuum thermal evaporated calcium has a sheet resistance ofabout 200 Ω/□, which meets the above requirement. A 20 Å thick Mg dopedwith 1% Ag has a sheet resistance of approximately 200 Ω/□. Sheetresistance may be controlled by material thickness, material type, ordoping concentration. For example, Mg doped with 10% Ag is much moreconductive than Mg doped with 1% Ag.

The simulation results of luminance uniformity of OLED device with auniform cathode and a 4-segment varied cathode are shown in FIG. 16.With four different cathode sheet resistance values, the non-uniformitymay be reduced to 0.76%.

Another technique to obtain matching resistance between the electrodesis to tune the OLED turn-on resistance to achieve luminance uniformity.For example, it is known that the thickness of the hole transportinglayer (HTL) has an impact on the device voltage. Therefore, by changingHTL thickness, the OLED turn-on resistance may be changed. An equivalentcircuit diagram is illustrated in FIG. 17, where the cathode has a zeropotential and the anode has a uniform resistance. For each OLED segment,the resistance due to the thick HTL, R_(HTL), can be extracted from thedevice.

In this case,

$\mspace{20mu}{V_{D,m}^{\prime} = {V_{D,{m - 1}}^{\prime} + {R_{a} \cdot {\sum\limits_{i = 1}^{m - 1}{A_{i}J_{i}}}}}}$$\mspace{20mu}{V_{D,m}^{\prime} = {\left. {V_{D,m} + {A_{m}{J_{m} \cdot R_{{HTL},m}}}}\Rightarrow{V_{D,m} + {A_{m}{J_{m} \cdot R_{{HTL},m}}}} \right. = {V_{D,{m - 1}} + {A_{m}{J_{m} \cdot R_{{HTL},{m - 1}}}} + {R_{a} \cdot {\sum\limits_{i = 1}^{{m - 1}\;}{A_{i}J_{i}}}}}}}$

For a uniform panel,J_(i)=JV_(D,m)=V_(D)

Which provides:AJ·R _(HTL,m) =AJ·R _(HTL,m-1) +AR _(a) ·mJ⇒R _(HTL,m) =R _(HTL,m-1) +mR _(a)

This provides the relationship of the adjacent HTL resistance.Therefore, by changing the thickness of the HTL for each segment, thevoltage of each OLED segment may be tuned to be equal across the panel.Note that, any means that controls or modifies the OLED turn-onresistance may be applied here, such as varying thickness of HIL,varying the ETL thickness, or any other suitable modification to thestructure of the device.

Fabrication techniques similar to that described with respect to FIG. 7may be used to form a gradient thickness across the panel. For example,the substrate may be tilted during deposition from an electrode materialsource, causing more electrode material to be deposited closer to oneend of a substrate than the other. A graded thickness also may beachieved in a linear vacuum deposition system where the materials may bedeposited with various thicknesses at different positions on thesubstrate when the substrate passes through the deposition chamber.Other techniques, such as those disclosed elsewhere herein, also may beused to fabricate an electrode having a gradient thickness.

EXPERIMENTAL

To determine the impact of this cathode resistance, a computersimulation program was developed to numerically calculate the voltage,current and luminance for each OLED segment, based on the equivalentcircuit in FIG. 8. The first step of this simulation is to calculate thevoltage and current of the first OLED under a target luminance, e.g.1,000 cd/m². This can be done by using current, voltage, and luminancedata measured from an equivalent small-area OLED pixel. The onlyassumption in the simulation is that the overall current that flows inand out of the whole OLED “string” is the same at both ends, and we setits initial value as n×I₁. With this current and I₁ known, the potentialof the next segment may be calculated, and thus the current andluminance. This process is repeated until the input current at the anodecontact is acquired. If this input current is the same as the outputcurrent at the cathode contact, then the solution is found. Otherwisethis process is repeated until the input and output currents are thesame.

Firstly, uniform resistance was introduced to cathode. The simulation isperformed on bottom-emission OLED. The sheet resistance of ITO is 15Ω/□. The cathode sheet resistance is varied from 0.01 Ω/□ to 20 Ω/□. TheOLED device is 2 cm long in one dimension where two electrode contactswere placed at opposite ends. The device is divided into 100 segments(i.e., n=100 in FIG. 8), with the first segment close to cathode and thelast to anode. The target luminance immediately adjacent to the cathodecontact at n=1 is set at 1,000 cd/m². The simulation results are plottedin FIG. 9, where the luminance levels across the OLED device fromcathode to anode are calculated for different cathode sheet resistances.As can be seen from the plot, when introducing cathode resistance, theluminance near the anode contact (n=100) decreases.

IR loss, power efficacy, and average luminance of the device with acathode resistance present were also calculated. The results aresummarized in Table 1. For a 2 cm, one-dimensional OLED device, withhighly conductive cathode (R_(k)=0.01 Ω/□), the luminance uniformity ofthe panel, defined as L_(min)/L_(max), is 88.2%. With proper tuning ofthe cathode resistance, the luminance uniformity is improved as R_(k)increases to R_(k)=20 Ω/□. In particular, when the cathode and anoderesistance are the same, i.e., R_(k)=R_(a)=15 Ω/□, uniformity has animproved value of 94.2%. Notably, even when additional resistance isintroduced in the device, the total IR loss is still very small, only1.34% when R_(k)=15 Ω/□. Since the resistance of the cathode is designedto be the same as that of the anode, the additional power loss due tointroducing a matched cathode resistance is the same as that of theanode. Thus the total power loss may be no more than twice the powerloss expected for a zero cathode resistance, which typically is arelatively very small number (0.71% in this example). However, if thecathode resistance increases beyond a threshold, for example to R_(k)=30Ω/□ in the present configuration, the luminance uniformity may beexpected to decrease. Thus it is expected that there is an optimizedvalue or value range for the cathode resistance so that a lowestnon-uniformity may be achieved. It has been found that this occurs whenboth resistances are within a small fraction of one another, andpreferably R_(k)=R_(a).

From the calculation above, it has been shown that by matching theresistance of two electrodes so as to achieve equal potential dropacross the panel, the luminance uniformity of the OLED device may beimproved.

TABLE 1 Device performance with different cathode resistance. Average Rkuniformity Efficacy IR loss Luminance [ohm/sq] [%] [lm/W] [%] [cd/m²]0.01 88.2 65.79 0.71 1045 5 91.2 65.81 0.94 1014 10 93.1 65.84 1.15 98615 94.2 65.86 1.34 961 20 91.4 65.87 1.52 937 30 86.2 65.9 1.85 894

In a further test, three OLED panels with the layout shown in FIG. 10were fabricated. The anode and cathode electrodes are placed on theopposite single edges of the panel. The dimension of the panel is 57mm×126 mm, which yields an active area of 72.1 cm². Device structures ofthe three panels are shown in FIGS. 11A-C. All three OLED panels includethe same anode structure (500 Å ITO/250 Å Ag/200 Å ITO) and the sameorganic layer stack, but different cathodes. Panel A (FIG. 11A) includesa 1000 Å Al layer; Panel B (FIG. 11B) includes a 500 Å Al layer; andPanel C (FIG. 11C) includes a 150 Å layer of Mg doped with 10% Ag. Thesheet resistances of these electrodes measured from four-point probesheet resistance meter are summarized in Table 2. Both Panels A and Chave one electrode that is more conductive than the other. Panel B hastwo electrodes with similar sheet resistances. Panel A has the mostconductive cathode among all three panels.

TABLE 2 Summary of sheet resistances of the electrodes used in Panel A,B and C. Al Al Mg:Ag 10% IZO/Ag/IZO 1000 Å 500 Å 150 Å 500/250/200 Å(Panel A) (Panel B) (Panel C) Sheet 2.1 0.7 1.9 22.2 Resistance [ohm/sq]

Photo-like images of the three Panels are shown in FIG. 12. Luminanceswere measured on each panel at 5 spots along the center line (indicatedin FIG. 12), using a luminance meter at normal incidence. All threepanels were driven at 1.8 mA/cm². The normalized luminance as a functionof position of all three panels is shown in FIG. 13. The luminanceuniformity is defined as L_(min)/L_(max), the ratio between the minimumand the maximum luminance. The luminance uniformity of Panel A is 81%,Panel B 89%, and Panel C 28%. It is apparent that even though thecathode of Panel A has a lower sheet resistance than that of Panel B,Panel B demonstrates a10% higher luminance unithrmity than Panel A. Thecathode of Panel C has the poorest conductivity among the three and alsonot comparable with its anode, therefore, Panel C has the lowestuniformity. This demonstrates the surprising result that the luminanceuniformity of a large-area OLED panel may be improved by addingresistance to cathode to match the resistances of anode.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. The present invention asclaimed may therefore include variations from the particular examplesand preferred embodiments described herein, as will be apparent to oneof skill in the art. It is understood that various theories as to whythe invention works are not intended to be limiting.

The invention claimed is:
 1. An organic light emitting devicecomprising: a first electrode; a second electrode; and an organicemissive layer disposed between the first electrode and the secondelectrode; wherein the first electrode has a gradient resistance in acurrent flow direction within the first electrode, and the secondelectrode has a gradient resistance in a current flow direction withinthe second electrode.
 2. The device of claim 1, wherein the firstelectrode has an electrical contact at the end of the first electrodewith the lowest resistance.
 3. The device of claim 1, wherein the devicehas at least one dimension of at least 2 cm.
 4. The device of claim 1,wherein an active area of the device is at least 50 cm².
 5. The deviceof claim 1, wherein the first electrode has a graded thickness in thecurrent flow direction.
 6. The device of claim 1, wherein a uniformpotential difference is generated between the first electrode and thesecond electrode when a current is applied to the device via the firstelectrode and the second electrode.
 7. The device of claim 1, whereinthe first electrode comprises a plurality of layers.
 8. The device ofclaim 7, wherein each layer of the plurality of layers has a differentarea.
 9. The device of claim 1, wherein the second electrode has agraded thickness in the current flow direction within the secondelectrode.
 10. The device of claim 1, wherein a uniform potentialdifference is generated between the first electrode and the secondelectrode when a current is applied to the device via the firstelectrode and the second electrode.
 11. The device of claim 1, whereinthe second electrode comprises a plurality of layers.
 12. The device ofclaim 11, wherein each layer of the plurality of layers has a differentarea.